Now I've only just put this PC together (as in last night) and at first it was running fine (I installed WinXP and Drivers etc.) Than I go to turn it on just now and it won't start, all that happens is that the fans go for about 1 sec than nothing everything stops working.
The powerpoints are fine, nothing wrong with monitor, nothing has changed since it was last running either

I think it might be a problem with the PSU not having enough power but i'm not 100% positive (If it's the PSU what do you reckon would be enough power?), or one of my friends said it could be the heatsink.

being all this is new and it worked fine through install, first i would make sure the ram is fully seated in the slot,, with the monitor unpluged from the video card, power unpluged from the power supply, i would use the cmos jumper, clear the cmos, see if it boots back up, if you were trying a overclock, or a setting was wrong by any other possibility, this should boot it back to the bios screen, where you should try the default settings, then see if the computer starts back up, you can set the bios to what you like later, and if you try a setting that doesnt work, just clear the cmos and the fun begins again.